Post-collisional Tectonic Setting Research Articles

Geochemical, isotopic and U-Pb geochronological investigation of the late Cretaceous Karaçayır carbonatite (Sivas, Turkey): Insights into mantle sources within a post-collisional tectonic setting

The Karaçayır carbonatite is located within the northeastern edge of the Central Anatolian Crystalline Complex, and represents one of the significant rare earth element (REE) mineralizations in Turkey. The carbonatite intrudes the associated syenite as dikes and veins, and is mainly composed of medium- to coarse-grained calcite, and lesser amounts of apatite, mica, amphibole, pyroxene, and magnetite. This study reports new and combined geochemical, radiogenic (Sr, Pb), stable (O and C), and for the first time B and Nd isotopic ratios and in-situ U-Pb age data from the Karaçayır carbonatite. The carbonate separates from the Karaçayır carbonatite are characterized by high Ba (30 to 484 ppm), Sr (613 to 9106 ppm) and total rare earth element (TREE) concentrations (172 to 2892 ppm), which are consistent with geochemical signatures for mantle-derived carbonate melts. The combined δ18OV-SMOW (+10.7 to +19.0 ‰), δ13CV-PDB (−2.9 to +1.8 ‰) and δ11B (−13.9 to −2.9 ‰) data for a majority of the carbonate separates investigated here in contrast indicate that they are the result of open system behavior involving contamination by the country rock (marble), and/or sub-solidus, low temperature alteration. Based on the combined stable and radiogenic isotopic signatures for the least altered/contaminated samples, the carbonatite magma at Karaçayır most likely represents a low-degree partial melt of an isotopically heterogeneous (metasomatized) mantle source containing EM (enriched mantle)-like components and recycled crustal carbon; the latter finding is consistent with those recorded for young (<300 Ma old) carbonatites worldwide. A weighted mean 206Pb/238U age of 69.3 ± 1.7 Ma is obtained for apatite from the Karaçayır carbonatite, and indicates its emplacement within a post-collisional tectonic setting during the late Cretaceous.

SHRIMP U–Pb ages, mineralogy, and geochemistry of carbonatite–alkaline complexes of the Sillai Patti and Koga areas, NW Pakistan: Implications for petrogenesis and REE mineralization

Carbonatites and related alkaline igneous rocks are important hosts of rare earth elements (REEs). Carbonatite–alkaline complexes in NW Pakistan represent rare examples of carbonatite emplaced in a collisional orogenic belt. However, the precise age, magmatic source, and petrogenesis of carbonatites emplaced in collisional settings and associated REE mineralization remain uncertain. The present study focuses on the origin and REE mineralization potential of alkaline–carbonatite complexes exposed in the Koga and Sillai Patti (SP) areas, NW Pakistan.The Koga carbonatites occur as minor plug-like intrusions in syenites. SHRIMP U–Pb dating of zircons from the Koga syenites, which are spatially associated with carbonatites, yield weighted mean ages of 283.6 ± 1.7(1σ), 281.9 ± 1.6(1σ), 282.6 ± 1.6(1σ), and 306.5 ± 3.7 Ma (1σ). The Koga syenites are characterized by high SiO2 (62.79–65.58 wt%) and Al2O3 (16.92–18.55 wt%) and low TiO2 (0.10–0.59 wt%) contents and are enriched in alkalis (Na2O > K2O). Sr and Nd isotopes from the Koga carbonatite complex are characterized by positive εNd(t) values (+3.10 to + 3.47) and low (87Sr/86Sr)i values (0.70233–0.70382), which are consistent with rift-related carbonatites.The SP carbonatites yield high total REE contents (1768–1920 ppm) and are enriched in light REEs (LREEs) relative to heavy REEs (HREEs; LREE/HREE = 20.0–20.7). The results of Sr–Nd isotope analyses show that the SP carbonatite has high Sr and low Nd isotopic ratios with negative εNd(t) values ranging from − 1.71 to − 2.21, variable 207Pb/206Pb ratios, and anomalous Pb–Sr and Pb–Nd patterns, suggesting Pb contamination during ascent of the primary carbonatite melts. Values of δ18OSMOW and δ13CPDB (‰) for the SP carbonates vary from + 14.45‰ to + 18.12‰ and from − 6.57‰ to − 9.70‰, respectively, which can be attributed to low-temperature alteration.On the basis of data for bulk-rock trace elements and Sr–Nd–Pb–C–O isotopes, it is concluded that the Koga carbonatite–alkaline complex was formed during the rifting and break-up of the northern margin of Gondwanaland. In contrast, carbonatite of the SP carbonatite–alkaline complex was probably generated by partial melting of sub-continental lithospheric mantle in a post-collisional tectonic setting. Although the SP carbonatite contains abundant REEs, it does not host significant REE mineralization owing to the lack of a mature sequence of evolutionary phases and to possible phosphate mineral saturation in the primary magmas, which caused REE depletion in the later carbonatitic magmas through fractional crystallization. Therefore, we conclude that carbonatites that are relatively rich in apatite but poor in fluorite and barite may not be prospective for REE mineralization.

Origin of the post-collisional carboniferous granitoids associated with the Azhahada Cu-Bi deposit in Inner Mongolia, Northeast China and implications for regional metallogeny

The Azhahada quartz vein-type Cu-Bi deposit is located in the Erenhot-East Ujimqin polymetallic metallogenic belt, on the western part of the Great Xing’an Range, Inner Mongolia, northeast China. The Cu-Bi veins are hosted by volcanic and metasedimentary rocks in the Devonian Niqiuhe Formation and Carboniferous felsic intrusions (monzogranite, syenogranite, and granite). Three hydrothermal ore stages (I-III) for Cu-Bi formation at Azhahada have been identified. Chalcopyrite and bismuthinite are the dominant ore minerals and mainly occur in vein stage II (quartz–pyrite ± chalcopyrite ± bismuthinite veins). New LA-ICP-MS zircon U-Pb dating indicates the monzogranite, syenogranite, and granite emplaced between 313.9 ± 1.7 and 308.4 ± 1.5 Ma. The monzogranite, syenogranite, and granite show the same geochemical affinity with I-type granites, which are characterized by high SiO2, K2O, and Al2O3 contents, low MgO, TiO2, and P2O5 contents, negative Eu anomalies (EuN/EuN* = 0.04–0.57), and strong enrichment in Rb, K, Pb, and Nd. Moreover, these granitic intrusions have low Mg# values of 22 to 39, low (87Sr/86Sr)i ratios (0.7023 to 0.7053), εNd(t) values between −0.8 and +0.4, positive εHf(t) values of +1.6 to +10.4, suggesting that their parental magmas were most likely derived from partial melts from the lower crust involving some mantle materials in a post-collisional tectonic setting. Sulfide samples from Azhahada have 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios similar to those of granitic intrusions. The silicon isotope compositions of the quartz samples (−0.3 to −0.1‰) are highly consistent with values of the granitic intrusions (−0.4 to −0.1‰), indicating that the silicon was derived from the granitic magma. Combined our whole-rock geochemical and Sr–Nd–Hf–Pb–Si isotopic data with previous investigation of regional tectonic evolutions, we suggest that the Azhahada granitoids genetically associated with the Cu-Bi mineralization were generated by the postorogenic lithospheric extension and magmatic underplating during the Late Carboniferous. A post-collisional extensional event coupled with the rising of hot mantle derived melts triggered partial melting of the lower crust, as well as provided metals (Cu and Bi).

Origin of high Ba-Sr granitoids at Chigou in central China and implications for Cu mineralization: Insights from whole-rock geochemistry, zircon U–Pb dating, Lu–Hf isotopes and molybdenite Re–Os systematics

The Chigou copper deposit is situated in the southern part of the Qinling orogenic belt, central China, where quartz diorite and quartz diorite porphyry are associated with porphyry-type copper mineralization. Copper generally occurs as veins, veinlets, and disseminations within-host porphyries. The zircon dates tightly constrain the timing of host porphyries to ca. 149–146 Ma, in agreement with the age of porphyry copper mineralization (146.1 ± 2.8 Ma) constrained by using molybdenite Re–Os dating. The calculated zircon ΔFMQ values for quartz diorite are −9.4 to +9.9 (averaging +4.3). The Chigou granitoids show similar geochemical compositions to high-Barium-Strontium (high Ba-Sr) granites, with high Ba (2256–3717 ppm) and Sr (737–1345 ppm), low Rb (86.2–129 ppm), U (1.9–4.7 ppm), Th (16.2–31.2 ppm), and Nb (11.4–19.1 ppm) concentrations, as well as lack of negative Eu anomalies. The concentrated εHf(t) values (−3.0 to +1.4), moderate Mg# values (41–52), and high (Hf/Sm)PMN and La/Sm ratios, but low Ba/Th, U/Th, and (Ta/La)PMN ratios indicate that ore-forming magmas derived from an enriched lithospheric mantle that had been metasomatized by subducted sediment-derived melts with a minor crustal contribution. Combined with previous studies, we suggest that the Chigou deposit formed in a post-collisional tectonic setting during the Late Jurassic.

Extensive mineralization in the eastern segment of the Xingmeng orogenic belt, NE China: A regional view

• There are four types of EXOB deposits: porphyry , skarn , hydrothermal vein, and epithermal types. • There are five stage mineralizations in EXOB: 480–440, 300–220, 200–165, 155–130, and 125–100 Ma. • The five metallogenic periods correspond to different geological setting. The eastern segment of the Xingmeng orogenic belt (EXOB) hosts numerous Mo, polymetallic Ag, Pb–Zn , Sn, Cu, Au (Ag), and rare-earth element (REE) deposits and is one of the most important polymetallic-ore regions globally. The mineralization type includes i) porphyry Cu (Mo, Au), Mo (Cu, Pb–Zn, W), Sn (Cu–Ag) deposits, ii) skarn Cu–Au, polymetallic Pb–Zn and Fe–Sn deposit, and iii) veined polymetallic Ag, Pb–Zn, Sn, Cu deposits and W, Mo deposits, with an REE deposit related to magmatic hydrothermal fluid; and iv) epithermal Ag and Au (Ag, Te) deposits, respectively. The mineralization-related rocks in the EXOB are mainly high-K calc-alkaline granites together with minor alkaline granites that were derived from the melting of juvenile crust. The mineralizations in the EXOB are typically related to magmatic-hydrothermal events. In this study we attempt a systematic analysis of the temporospatial distribution of ore deposits in the EXOB, in order to constrain the geodynamic setting of mineralization. Based on available geochronological data, we suggest five mineralization stages for the EXOB at 480–440, 300–220, 200–165, 155–130, and 125–100 Ma, respectively. The porphyry Cu–Mo mineralization occurred during the early Paleozoic (480–440 Ma). The extensive Mo (W, Sn), and Ag (Pb–Zn) polymetallic mineralization was formed during the Mesozoic. The Ordovician–Triassic mineralization (480–220 Ma) is related to the Paleo-Asian Ocean tectonism and developed in an island arc setting. The Permian–Triassic mineralization (300–220) was formed in a post-collisional tectonic setting related to the development of Hegenshan suture. The early–middle Jurassic deposits (200–165 Ma) are related to the subduction of the Paleo-Pacific oceanic plate on the East Asian continental margin. The Late Jurassic–Early Cretaceous deposits (155–130 Ma) were formed in an extensional setting related to rollback of the Paleo-Pacific Plate. The Jurassic–Cretaceous mineralization (200–100 Ma) in the northern segment of the Da Hinggan Mountains is associated with the subduction and post-collisional tectonic setting of the Mongol–Okhotsk oceanic plate. The latest stage of epithermal deposits (125–100 Ma) were developed in a continental margin back-arc setting associated with subduction of the Paleo-Pacific Plate beneath the Eurasia.

The Neoproterozoic to Triassic tectonic evolution of Jangbong Island in the northwestern Gyeonggi Massif on the Korean Peninsula

Jangbong Island is located in the coastal area of the northwestern Gyeonggi Massif, and its basement is composed of Paleoproterozoic gneiss. The basement is covered by Neoproterozoic metasedimentary rocks such as pelitic schist, calc-schist, and quartz schist. These rocks are intruded by mafic dikes metamorphosed under amphibolite facies conditions. In the study area, two different metamorphic conditions are confirmed: the first was intermediate-P/T peak metamorphism (680-630 °C/8.6-7.3 kbar), and the second was low-P/T metamorphism (600-560 °C/5.7-3.1 kbar) showing a clockwise P-T path. All rocks in the studied area are intruded by Triassic gabbro and granite. Age measurement using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) yields intrusion ages of 917 ± 33 Ma and 873 ± 32 Ma for metabasites. Two different metamorphic ages are also obtained from them: 255 ± 12 ~ 254.3 ± 1.3 Ma and 231.5 ± 1.9 ~ 229.8 ± 1.4 Ma for the first and second metamorphic episodes, respectively. The Triassic gabbro yields an intrusion age of 229.1 ± 0.57 Ma and formed in a post-collisional tectonic setting with Triassic granite. The geochemical analysis confirms that Jangbong Island metabasites formed in a within-plate tectonic setting and are all alkaline basalts and mostly tholeiitic. The 917-873 Ma rift-related mafic intrusion is well matched with the Neoproterozoic rift-related mafic intrusions in the northern Gyeonggi Massif and along the southeastern margin of the North China Craton (NCC). The 255-254 Ma intermediate-P/T peak metamorphism is well matched with the collisional event in the northern Gyeonggi Massif, and the 231-230 Ma low-P/T metamorphism and 229 Ma post-collisional igneous activity are well correlated with post-collisional events in the northern Gyeonggi Massif. Together with previous data, these data support the correlation between the northern Gyeonggi Massif and the NCC.

A-type granites induced by a breaking-off and delamination of the subducted Junggar oceanic plate, West Junggar, Northwest China

The A-type granites with highly positive εNd(t) values in the West Junggar, Central Asian Orogenic Belt (CAOB), have long been perceived as a group formed under the same tectonic and geodynamic setting, magmatic sourceq and petrogenetic model. Geological evidence shows that these granites occurred at two different tectonic units related to the southeastern subduction of Junggar oceanic plate: the Hongshan and Karamay granites emplaced in the southeast of West Junggar in the Baogutu continental arc; whereas the Akebasitao and Miaoergou granites formed in the accretionary prism. Here the authors present new bulk-rock geochemistry and Sr-Nd isotopes, zircon U-Pb ages and Hf-O isotopes data on these granites. The granites in the Baogutu continental arc and accretionary prism contain similar zircon εHf(t) values (+10.9 to +16.2) and bulk-rock geochemical characteristics (high SiO2 and K2O contents, enriched LILEs (except Sr), depleted Sr, Ta and Ti, and negative anomalies in Ce and Eu). The Hongshan and Karamay granites in the Baogutu continental arc have older zircon U-Pb ages (315–305 Ma) and moderate 18O enrichments (δ18Ozircon=+6.41‰–+7.96‰); whereas the Akebasitao and Miaoergou granites in the accretionary prism have younger zircon U-Pb ages (305–301 Ma) with higher 18O enrichments (δ18Ozircon=+8.72‰–+9.89‰). The authors deduce that the elevated 18O enrichments of the Akebasitao and Miaoergou granites were probably inherited from low-temperature altered oceanic crusts. The Akebasitao and Miaoergou granites were originated from partial melting of low-temperature altered oceanic crusts with juvenile oceanic sediments below the accretionary prism. The Hongshan and Karamay granites were mainly derived from partial melting of basaltic juvenile lower crust with mixtures of potentially chemical weathered ancient crustal residues and mantle basaltic melt (induced by hot intruding mantle basaltic magma at the bottom of the Baogutu continental arc). On the other hand, the Miaoergou charnockite might be sourced from a deeper partial melting reservoir under the accretionary prism, consisting of the low-temperature altered oceanic crust, juvenile oceanic sediments, and mantle basaltic melt. These granites could be related to the asthenosphere’s counterflow and upwelling, caused by the break-off and delamination of the subducted oceanic plate beneath the accretionary prism Baogutu continental arc in a post-collisional tectonic setting.©2022 China Geology Editorial Office.

Timing and Nd-Hf isotopic mapping of early Mesozoic granitoids in the Qinling Orogen, central China: Implication for architecture, nature and processes of the orogen

The Qinling orogen, one of the most important orogens in Asia, belongs to the northeastern part of the Tethyan orogen. The architecture and processes of the Qinling orogen remain controversial. In this study, we present 15 new zircon U–Pb ages, 20 whole-rock geochemical and 46 Sm-Nd isotopic analyses, and 30 zircon Lu–Hf isotopic data for early Mesozoic granitoids in this orogen, combining with data from literature, to delineate the crustal architecture and processes of the orogen. A total of 181 zircon U–Pb ages show three phase (252–230, 230–198, and 190–185 Ma) of granitoids. The first-phase granitoids occur mainly in the westernmost segment of the orogen and formed in a subduction setting during the closure of the Mianlue Ocean (a northern branch of the Paleo-Tethyan Ocean). The second- and third-phase granitoids, distributed in the middle to eastern parts of the Qinling orogen, were generated in late syn-collisional and post-collisional tectonic settings, respectively. Whole-rock Nd and zircon Hf isotopic mapping of these granitoids yield six and seven isotopic provinces, respectively. These provinces display that the southern margin of the North China Block and the northern margin of the South China Block are dominated by ancient deep crust, that is, early Paleoproterozoic (2.3–1.8 Ga) and late Paleoproterozoic (∼1.7 Ga) components, respectively. By way of camparison, the North Qinling contains younger Mesoproterozoic [eNd(t) = −10.7 to −0.2; TDM = 1.4–1.0 Ga] basement, evidencing that it is an independent terrane different from the North China Block. The isotopic mapping also reveals a deep-seated NNE–SSW-trending zoned architecture that is approximately perpendicular to the WNW–ENE-trending of the orogen. This provides new evidence for the “Spaghetti Junction model” for the Qinling orogen. The old Nd (2.2–1.0 Ga, mostly 2.0–1.2 Ga) and Hf (2.3–0.8 Ga, mostly 2.0–1.2 Ga) model ages indicate that the continental growth in this orogen occurred mainly during the Paleoproterozoic and Mesoproterozoic, with only minor amounts of juvenile [eNd(t) = ∼0, TDM = ∼0.1 Ga] continental growth along the Shangdan and Mianlue sutures. These characteristics suggest that the Qinling orogen is dominantly formed by the collision of ancient continental blocks, distinct from some typical accretionary orogens, such as the Central Asian Orogenic Belt with voluminous juvenile crust.

Thermobarometric and geochronologic constraints on the emplacement of the Neoproterozoic Evate carbonatite during exhumation of the Monapo granulite complex, Mozambique

Fluid inclusion and mineral thermobarometric study supplemented by U-Th-total Pb monazite dating has been carried out in country rocks of the Evate carbonatite, which is an example of a Neoproterozoic (~590 Ma) orogenic carbonatite formed within a post-collisional tectonic setting in the Monapo granulite complex of NE Mozambique. The fluid inclusion analysis of a 573 ± 13 Ma old fenite subjacent to the Evate carbonatite revealed primary, high-density to superdense CO2-rich fluids (>1 g.cm−3) with minor admixtures of N2 (0–17.5 mol%) and CH4 (0–1.1 mol%) accompanied by alkalic hydrocarbonate melts and aqueous fluids with minor sulphates. Isochores of CO2 inclusions intersected by isopleths calculated from Ti-in-quartz and Zr-in-rutile thermometers indicate fenite formation at 480–530 °C and 2.1–5.4 kbar. The CO2-rich gas of the fenite-forming fluids/melts is similar to the anhydrous CO2 gas with traces of N2 (<1.5 mol%) trapped in granulites and paragneisses of the Monapo Complex during granulite-facies metamorphism (~850 °C, ~9 kbar) and superimposed retrogression under amphibolite-facies conditions (~550 °C, ~5 kbar) within the time interval between 680 and 590 Ma (Cryogenian to Ediacaran). Peak metamorphic conditions within the Monapo Complex are diagnostic of a hot orogen similar to that in other Pan-African Neoproterozoic HT/UHT granulite terranes. The exhumation PT trajectory of the Monapo Complex diverted from a „near-isochoric“-shaped curve to near-isothermal (~500 °C) decompression at ~590 Ma coeval with the intrusion of the Evate carbonatite into the disrupted continental crust. The pressure fluctuations during the concomitant fenitization were reflected in densities of carbonic inclusions, ranging between 1.178 and 0.773 g.cm−3. Secondary, low-density N2-rich inclusions (82–99 mol% N2, 0.8–13.7 mol% CH4, 0–4.5 mol% CO2) homogenized to vapour at temperatures below −173 °C recorded the fenite cataclasis at temperatures below 400 °C and very low pressures. The probable maximum burial depth of the Evate carbonatite, ~20 km, is estimated from the maximum fluid pressure of 5.4 kbar attributed to lithostatic load of the crust with an average density of 2.75 g.cm−3.

Geochronology and petrogenesis of the composite Zuluhong Granite, North Xinjiang Province of China: Implications for the crust-mantle interaction and continental crustal growth in Western Tianshan Orogen

The mineralogy, whole rock geochemistry, zircon U-Pb-Hf isotopic compositions, and Sr–Nd isotopes were analysed from the composite Zuluhong Granite at the Alataw Mountains in NW China. The aim is to determine the granite's nature and tectonic setting, and gain a better understanding of the continental growth of the Western Tianshan Orogen (WTO), which is commonly regarded as part of the Central Asian Orogen (CAO). The Zuluhong Granite is divided into a monzogranite and porphyritic granodiorite. The monzogranite yields a zircon U–Pb date of 320 ± 2 Ma interpreted as its emplacement age, and the porphyritic granodiorite yields a zircon U–Pb date of 309 ± 1 Ma interpreted as its emplacement age. The monzogranite is characterised by its high SiO2 content (72.7–74.4 wt%), moderate 10,000 × Ga/Al values(2.59–2.81), strongly negative Eu anomaly (0.39–0.43), and relatively low zircon saturation temperatures (Tzr) of 726–807 °C, which are indicative of a highly fractionated monzogranite. The combined arc-like geochemical signatures (e.g. enrichment in LILE and LREE relative to HFSE, negative anomalies in Nb and Ta), and relatively high positive εNd(t) values of +4.4 to +4.8, indicate that the monzogranite originated from the partial melting of arc rocks in the lower crust. In contrast, the granodiorite has higher Al2O3 assays averaging 16 wt% and Sr/Y ratios averaging 40, and lower Yb assays of 1.38–1.49 ppm with a weak Eu anomaly, which are characteristics of adakite-like rocks. The combination of the granodiorite's relatively lower SiO2 content of 63.7–65.3 wt%, higher Mg# values of 53–57, higher εHf(t) values of +10.9 to +13.4, and complex oscillatory zoned plagioclase phenocrysts are indicative of a source derived from material derived from the mantle and lower crust. Our data on the Zuluhong Granite, coupled with the results of previous studies of the Late Carboniferous to Early Permian magmatism in the WTO, indicate that a regional tectonic transition from compression to extension took place in the CAO during the end of the Early Carboniferous (ca. 320 Ma). Furthermore, substantial post-collisional reworking of the Precambrian rocks along the northeastern margin of the Yili Terrane is more pronounced than the West Junggar Terrane to the east. The generation of widespread magmatism and continental growth in the CAO in a post-collisional tectonic setting including the recycling of earlier juvenile-arc complexes, and underplating of mantle-derived magma.

Zircon U–Pb Ages and Geochemistry of Granite Porphyries in the Yangla Cu Deposit, SW China: Constraints on Petrogenesis and Tectonic Evolution of the Jinshajiang Suture Belt

Located in the eastern part of the Tethyan tectonic domain, the Jinshajiang Suture Belt (JSB), northwestern Yunnan, China, is notable for its large-scale distribution of Jurassic to Triassic granitoids that are genetically related to the evolution of the Paleo-Tethys Ocean and polymetallic mineralization. In this study, geochronological and geochemical analyses were conducted on three samples of these granite porphyries (GPs) using laser ablation inductively coupled plasma mass spectrometry and zircon U–Pb aging to reveal ages of 213±15, 198.4±8.6, and 195.3±6.4 Ma, respectively. These ages are younger than the emplacement ages of the granodiorites, at 208–239 Ma, suggesting that magmatic activities in the Yangla mining district likely continued for ~44 Ma. These GPs are rich in large-ion lithophile elements such as Rb, Ba, Th, U, K, and La but are deficient in high field strength elements such as Ta, Nb, Ce, Zr, Hf, and Ti. Significant Pb enrichment and P depletion were noted, as were varying degrees of metallogenic element enrichment in the order of Cu&gt;Pb&gt;Zn. The total content of rare earth elements (ΣREEs) of the GPs is in the range of 50.41–127.27 ppm and the LREE/HREE ratio is in the rage of 4.46–10.54. The GPs are rich in LREEs, with a high degree of differentiation noted between the LREEs and HREEs. The δEu (EuN/Eu∗) and δCe (CeN/Ce∗) values, at 0.53–0.86 and 0.79–0.98, indicate weak and slightly weak negative anomalies, respectively. The geochemical characteristics of the GPs indicate that these bodies are slightly metaluminous to peraluminous S-type granites in a calc–alkaline series that formed in a late-collisional or postcollisional tectonic setting. Three-component mixing of magmas including those of upper crust, lower crust, and mantle materials in addition to subsequent partial melting could have been responsible for the generation of these GPs in an epithermal low-pressure setting at &lt;5 kbar.

A Grenvillian magmatic almandine garnet-bearing ferroan granite intrusion in the Chhotanagpur Gneissic complex, Eastern India: Petrology, petrochemistry, petrogenesis and geodynamic implications

We report for the first time magmatic Ca-rich (CaO = 7–11.3 wt%) almandine garnet-bearing ferroan granites. The almandine (Alm66.6Grs24.2Pyr4.9Spes3.9Uvr0.1) garnet-bearing Tilaboni granite pluton was emplaced in a major regional shear zone of Chhotanagpur Gneissic Complex, Eastern India. Garnets show partial dissolution, corona, and symplectitic textures. Petrography and composition of minerals suggest the garnets are magmatic. Mineral chemistry of these garnets differs from those of calc-alkaline I-type, S-type, A-type, mantle-type, and metamorphic garnets. Geochemically, the host granites show high 10,000Ga/Al ratios (average 3.2), high K2O (average 4.91 wt%), high total alkalis (average 7.5 wt%), high Ta + Yb (average 10.2 ppm), high Ce + Nb + Zr + Y (average 668 ppm), high Ce/Yb (average 27.3) and strong negative Eu-anomalies (average 0.3). These granites are classified as ferroan, calc-alkalic to alkalic, metaluminous to weakly peraluminous, and highly fractionated I-type. Early crystallization of magnesian amphiboles/biotites enriched the FeO in the derivative melts. The granite magma had high liquidus temperature (800–950 °C), low oxygen fugacity (ΔQFM = +1 to −1.6), and solidified at around 5 to 6 kb pressure (Al-in hornblende barometers). Pseudosection modelling shows that the garnets crystallized from a hydrous melt (H2O = 6–9 wt%) at around 760 °C temperature, 6 kb pressure, and fO2–15 log unit (bar). The pluton emplaced in a post-collisional tectonic setting. Low Mg# (average 0.24), low Nb/U (average 9.8), and Ce/Pb (average 7.1) ratios but high Th/U (average 9.8) ratios of the Tilaboni granites strongly suggest their crustal source. The granite magma was derived by the 20–40% partial melting of an old high-K high alumina shoshonitic hornblende granulite protolith at around 7 kb. The shear zone facilitated the fast upward movement of the magma and incomplete dissolution of the garnets before solidification at lower pressure.

Post-volcanic activities in the Early Miocene Kırka-Phrigian caldera, western Anatolia – caldera basin filling and borate mineralization processes

ABSTRACT The formation of large, economic borate deposits requires a boron-rich source, the means of transporting and concentrating the boron in a restricted environment, and mechanisms for the preservation of the deposit. There are several Miocene basins in western Turkey containing world-class borate reserves, with mineralization present as stratabound deposits in volcano-sedimentary successions. Although it is well-documented that the conditions required to form and preserve large borate deposits are most common in post-collisional tectonic settings (of which western Anatolia is a prime example), recent advances in the understanding of extensional tectonics and volcanism in this region, make it possible to gain fresh insights into their formation. Here, we suggest that formation of one of the largest borate deposits in the world was intimately related to the recently recognized Kırka-Phrigian caldera that lies in the northernmost part of the Miocene Eskişehir–Afyon volcanic field. Following caldera collapse, the basin filled with lacustrine sediments and volcaniclastic deposits with the boron mineralization concentrated in two main sub-basins: Sarıkaya and Göcenoluk. The close spatial and temporal relationship between borate deposition and the vast Early Miocene ignimbrite deposits that surround the caldera (and contain high levels of elements associated with mineralization) strongly suggest that the ignimbrites were the major source of boron. The boron was transported by geothermal fluids and post-volcanic gases that vented into warm water at the base of the caldera-paleolake system and was then concentrated during cycles of sedimentation and evaporation, with most of the mineralization concentrated along a N-S striking fault system.

Zircon U–Pb geochronology and geochemistry of granitic rocks in central Mongolia

The Central Asian Orogenic Belt had been formed by amalgamation of voluminous subduction–accretionary complexes during the Late Neoproterozoic to the Mesozoic period. Mongolia is situated in the center of this belt. This study presents new zircon U–Pb geochronological, whole-rock major and trace element data for granitoids within central Mongolia and discusses the tectonic setting and evolution of these granitic magmas during their formation and emplacement. The zircon U–Pb ages indicate that the magmatism can be divided into three stages: the 564–532 Ma Baidrag granitoids, the 269–248 and 238–237 Ma Khangai granitoids. The 564–532 Ma Baidrag granitoids are adakitic, have an I-type affinity, and were emplaced into metamorphic rocks. In comparison, the 269–248 Ma granitoids have high-K, calc-alkaline, granodioritic compositions and are I-type granites, whereas the associated the 238–237 Ma granites have an A-type affinity. The 564–532 Ma Baidrag and 269–248 Ma Khangai granitoids also both have volcanic arc-type affinities, whereas the 238–237 Ma granites formed in a post-collisional tectonic setting. These geochronological and geochemical results suggest that arc magmatism occurred at the 564–532 Ma which might be the oldest magmatic activity in central Mongolia. Between the Baidrag and the Khangai, there might be paleo-ocean and the oceanic plate subducted beneath the Khangai and produced voluminous granite bodies during the 269–248 Ma. After the closure of the paleo-ocean, the post collisional granitoids were formed at the 238–237 Ma based on the result of later granitoids in the Khangai area.

Paleoproterozoic postcollisional metamorphic and igneous activities in the Jinan area of the Jiao-Liao-Ji Belt in the North China Craton and their tectonic implications

The Jiao-Liao-Ji Belt (JLJB) is a Paleoproterozoic collisional belt between the Longgang Block in the northeastern North China Craton and the Nangrim Massif on the Korean Peninsula. The Jinan area, located in the northeastern JLJB, was intruded by monzogranite at 2151 ± 6 Ma and then intruded again by porphyritic granites at 1842 ± 5 Ma in a postcollisional tectonic setting. The monzogranite and an associated gabbro were metamorphosed into metamonzogranite and metabasite at 1868 ± 11 Ma–1846 ± 3 Ma. Pelitic granulites occur together with the porphyritic granites. The pelitic granulites have peak metamorphic mineral assemblages of garnet + biotite + plagioclase + K-feldspar + sillimanite + quartz + ilmenite and underwent retrograde metamorphism, as evidenced by the presence of cordierite and muscovite. Pseudosection analysis of the granulites yields peak metamorphic conditions of ~8.5–7 kbar and 800–760 °C, and conventional geothermobarometry analysis of garnet rims and adjacent minerals yields retrograde metamorphic conditions of ~5–3.5 kbar and 740–680 °C. Dating of monazite from the two granulites demonstrates that peak metamorphism occurred between 1840 ± 2 Ma and 1847 ± 4 Ma. These results, together with the findings of previous studies, indicate that in the Liaodong and Jinan areas of the JLJB, igneous and metamorphic activities occurred together at ~1.85 Ga in a postcollisional tectonic setting, suggesting that collision between the Longgang Block and the Nangrim Massif occurred before ~1.85 Ga.

Early Permian to Late Triassic tectonics of the southern Central Asian Orogenic Belt: geochronological and geochemical constraints from gabbros and granites in the northern Alxa area, NW China

AbstractSituated between the North China Craton to the east and the Tarim Craton to the west, the northern Alxa area in westernmost Inner Mongolia in China occupies a key location for interpreting the late-stage tectonic evolution of the southern Central Asian Orogenic Belt. New LA-ICP-MS zircon U–Pb dating results reveal 282.2 ± 3.9 Ma gabbros and 216.3 ± 3.2 Ma granites from the Yagan metamorphic core complex in northern Alxa, NW China. The gabbros are characterized by low contents of Si, Na, K, Ti and P and high contents of Mg, Ca, Al and Fe. These gabbros have arc geochemical signatures with relative enrichments in large ion lithophile elements and depletions in high field strength elements, as well as negative εNd(t) (−0.91 to −0.54) and positive εHf(t) (2.59 to 6.37) values. These features indicate that a depleted mantle magma source metasomatized by subduction fluids/melts and contaminated by crustal materials was involved in the processes of magma migration and emplacement. The granites show high-K calc-alkaline and metaluminous to weakly peraluminous affinities, similar to A-type granites. They have positive εNd(t) (1.55 to 1.99) and εHf(t) (5.03 to 7.64) values. These features suggest that the granites were derived from the mixing of mantle and crustal sources and formed in a postcollisional tectonic setting. Considering previous studies, we infer that the final closure of the Palaeo-Asian Ocean in the central part of the southern Central Asian Orogenic Belt occurred in late Permian to Early–Middle Triassic times.

Magmatic evolution and related W–Mo mineralization in the Larong deposit, eastern Tibet: Evidence from zircon U–Pb ages, geochemistry and Sr–Nd–Hf isotopes

The Late Cretaceous (ca. 92 Ma) Larong porphyry W–Mo deposit (39.44 Mt WO3 @ 0.1541%) is the first-discovered giant W–Mo deposit in the Leiwuqi-Zogang metallogenic belt, Sanjiang region. Multi-stage granitoids including the dacite porphyry (DP), granodiorite porphyry (GP), biotite granite porphyry (BGP), monzogranite porphyry (MP), biotite granite (BG), and granite aplite (GA) have been identified in the Larong deposit. However, the magmatic evolution and related W–Mo mineralization in this deposit are still poorly understood. LA–ICP–MS zircon U–Pb dating of the DP, GP, BGP, MP, BG, and GA yield weighted mean 206Pb/238U ages of 214.0 ± 0.7 Ma, 213.8 ± 1.3 Ma, 104.4 ± 0.9 Ma, 93.9 ± 1.3 Ma, 91.7 ± 0.5 Ma, and 36.2 ± 0.6 Ma, respectively. The Late Triassic DP and GP should be classified as peraluminous (A/CNK values of 0.95–1.57) S-type granitoids. They are characterized by pronounced negative εNd(t) values (−10.01 to −13.92) and εHf(t) values (−8.5 to −16.6) with old TDM2(Nd) and TDM2(Hf) ages (1.81–2.12 Ga and 1.79–2.30 Ga, respectively), indicating that the DP and GP were mainly derived from partial melting of ancient crustal materials after break-off of the Paleo-Tethyan oceanic slab. The Late Cretaceous BGP, MP, and BG and Late Eocene GA have low A/CNK ratios (0.92–1.06), but high SiO2 (70.57%–77.34%) and differentiation index (DI = 87.55–98.06), similar to the characteristics of highly fractionated I-type granite. Detailed elemental (e.g., depletion in Nb, Ta, Ti, Sr, Ba, Eu, and P) and isotopic data (87Sr/86Sr(i) = 0.71192–0.72932; εNd(t) = −11.81 to −9.11; εHf(t) = −10.4 to 1.4) demonstrate that they were the result of magma mixing between ancient crust-derived melts and mantle-derived melts. The BGP was produced during the final stage of Bangong–Nujiang Tethyan ocean (BNTO) subduction, the MP and BG were formed under a post-collisional tectonic setting related to slab break-off of the BNTO, and the GA was likely associated with delamination of the thickened eclogitic lower crust together with lithospheric mantle. The W–Mo mineralization in the Larong deposit is genetically associated with the MP, with four favorable factors that can account for the formation of this deposit: (1) high W background, (2) multi-stage magmatism, (3) highly evolved, relatively oxidized, and F-rich magmatic system, and (4) sufficient calcium obtained by intense fluid-rock interaction.

Origin of post-collisional A-type granites in the Mahakoshal Supracrustal Belt, Central Indian Tectonic Zone, India: Zircon U-Pb ages and geochemical evidences

There are several A-type granitic plutons in the central and western parts of the Mahakoshal Supracrustal Belt (MSB), India. We report new whole-rock elemental and Nd isotope data along with zircon U-Pb ages on these granites. The ID-TIMS zircon U-Pb ages of these granites are 1645 ± 25 Ma, 1695 ± 9 Ma, and 1796 ± 17 Ma. These granites are metaluminous to weakly peraluminous, alkali-calcic to calc-alkalic and ferroan in composition. They are characterized by enrichment in REEs (except Eu) and HFSEs (Zr, Y), and depletion in LILEs (Ba, Sr). They have high Ga/Al (2.6–5.4) ratios and elevated magmatic temperatures (≥900 °C). Elemental ratios such as Y/Nb, Nb/Ta, Th/Nb, La/Nb and Ce/Pb coupled with negative Nb, Sr, Eu, and Ti anomalies suggest their derivation from crustal sources. They have very low εNd(t) values (−11.4 to −5.6) and older Nd model ages (TDM: 2481–3225 Ma) confirming their crustal derivation. We propose that partial melting of the Archean tonalitic-granodioritic crust of basement Bundelkhand Craton in low-pressure high-temperature conditions generated the parental magma of these granites. Their A2-type affinity (Y/Nb ˃ 1.2) further suggests their emplacement in an extensional environment related to post-collisional tectonic setting.

Subduction-related hybridization of the lithospheric mantle revealed by trace element and Sr-Nd-Pb isotopic data in composite xenoliths from Tallante (Betic Cordillera, Spain)

Ultramafic xenoliths are rarely found at convergent plate margins. A notable exception is in the Betic Cordillera of southern Spain, where the eruption of xenolith-bearing alkaline basalts during the Pliocene post-dated the Cenozoic phase of plate convergence and subduction-related magmatism. Mantle xenoliths of the monogenetic volcano of Tallante display extreme compositional heterogeneities, plausibly related to multiple tectono-magmatic episodes that affected the area. This study focuses on two peculiar composite mantle xenolith samples from Tallante, where mantle peridotite is crosscut by felsic veins of different size and mineralogy, including quartz, orthopyroxene, and plagioclase. The veins are separated from the peridotite matrix by an orthopyroxene-rich reaction zone, indicating that the causative agents were alkali-rich hydrous silica-oversaturated melts, which were likely related to recycling of subducted continental crust components. The present study reports new and detailed major and trace elements and Sr-Nd-Pb analyses of the minerals in the composite Tallante xenoliths that confirm the continental crust derivation of the metasomatic melts, and clarifies the mode in which subduction-related components are transferred to the mantle wedge in orogenic areas. The particular REE patterns of the studied minerals, as well as the variation of the isotopic ratios between the different zones of the composite xenoliths, reveal a complex metasomatic process. The distribution of the different elements, and their isotope ratios, in the studied xenoliths are controlled by the mineral phases stabilised by the interaction between the percolating melts and the peridotitic country rock. The persistence of marked isotopic heterogeneities and the lack of re-equilibration suggest that metasomatism of the sub-continental lithospheric mantle occurred shortly before the xenolith exhumation. In this scenario, the studied xenoliths and the metasomatic processes that affected them may be representative of the mantle sources of mafic potassic to ultrapotassic magmas occurring in post-collisional tectonic settings.

Geochemical discrimination between fertile and barren Eocene potassic porphyries in the Jinshajiang Cu–Au–Mo metallogenic belt, SW China: Implications for petrogenesis and metallogeny

The Jinshajiang potassic porphyry metallogenic belt represents one of the most important Chinese intrusive belts that hosts Eocene Cu–Au–Mo porphyry systems including the Narigongma Mo, Yulong Cu, and Beiya Au deposits. This metallogenic belt also contains several uneconomic potassic intrusions, such as the Jianchuan, Shigu and Binchuan granite porphyries. Bulk-rock geochemical compositions together with Sr–Nd–Pb isotope ratios, and zircon Hf isotope and trace element compositions are compiled and interpreted to reveal the fundamental differences leading to the different fertility of these potassic porphyries at Jinshajiang. The economic Narigongma, Yulong, Beiya, and barren Jianchuan, Binchuan, Shigu granite porphyries are all formed in the same post-collisional tectonic setting, and all exhibit high SiO2 (61–65 wt%), and total alkali contents (K2O + Na2O > 8 wt%). Based on low Ce contents (<10), FeOT/MgO ratios (<100) ratios and A/CNK values (<1.1), these granite porphyries are classified as potassic I-type granites. High (87Sr/86Sr)i ratios (0.70669 to 0.70786), low bulk-rock εNd(t) values (−8.7 to −0.7), MgO (0.2–1.4 wt%), Cr (0.1–36.4 ppm) and Ni (0.4–26.8 ppm), and radiogenic Pb isotopic compositions suggest that the granite porphyries, both fertile and barren, were most likely generated by partial melting of thickened lower crust with the addition of mantle-derived magmas. However, the fertile Narigongma, Yulong and Beiya granite porphyries have low Y (7.8–16.0 ppm) and Yb (0.6–1.5 ppm) contents, high (La/Yb)N ratios (31.3–98.9) and overall steeper REE profiles, reflecting residual garnet during partial melting of thickened lower crust, whereas the barren Jianchuan, Shigu and Binchuan granite porphyries have high Y (12.3–19.4 ppm) and Yb (1.1–2.5 ppm) contents, low (La/Yb)N ratios (25.2–44.3) and flat HREE patterns, indicating a garnet-poor magma source. Garnet that remains as a residual mineral during partial melting of thickened lower crust can drive magma oxidation, as shown by higher zircon Ce4+/Ce3+ ratios (averaging 75.6) and high fO2 values (averaging ΔFMQ = +3.9) of the fertile Yulong and Beiya granite porphyries than those of the barren Shigu and Jianchuan granite porphyries with average Ce4+/Ce3+ = 18.2 and ΔFMQ = +0.6. Therefore, magma source played an important role in determining the fO2 of the granite porphyry, thus controlling its fertility for Cu–Au–Mo mineralization.